Abstract
We used the method of measuring potential difference between two Ag|AgCl electrodes immersed directly into electrolyte solution with lower concentration and at different distances from membrane. The bacterial cellulose membrane was placed in horizontal plane in the membrane system with configurations with higher NaCl concentration and density under (A) and over the membrane (B). In both configurations at the initial moment the voltage between electrodes amounted to zero. After turning off mechanical stirring of solutions, in configuration A we observed the monotonic increase and next stabilization of voltage while in configuration B after short time dependent on the initial quotient of NaCl concentrations on the membrane we observed appearance of pulsations of measured voltage and gradual decrease of mean voltage over time. Smooth changes of voltage are connected with diffusional reconstruction of Concentration Boundary Layers (CBLs) while fast increase and subsequent pulsations of voltage are connected with the appearance of hydrodynamic instabilities (gravitational convection) near membrane imposed on diffusive reconstruction of thin layer. The time needed for the appearance of hydrodynamic instabilities in CBL depended nonlinearly on the initial ratio of electrolyte concentrations on the membrane.
Highlights
A membrane separates the volumes of a system and allows for controlling an exchange of solvent and dissolved solutes
In our study we extend the method of voltage measurement in the membrane systems [18,19,20] to the case with both electrodes in one chamber with lower ions concentration
Configurations of the membrane system with the membrane in horizontal plane are characterized by diffusive Concentration Boundary Layers (CBLs) reconstruction at the membrane surfaces in the direction of gravitational acceleration
Summary
A membrane separates the volumes of a system and allows for controlling an exchange of solvent and dissolved solutes. One of the membranes with electrically neutral structure and suitably high porosity making it possible to study convection phenomenon is the bacterial cellulose membrane This membrane is frequently used in medicine as wound cover because of its high hydrophilic properties, good adhesion to skin and efficient transport of water and small molecules [11,12,13]. For B configuration of the membrane system the time needed for reconstruction of suitably high gradients of densities in CBLs to appear hydrodynamic instabilities was measured and calculated from the model and the concentration Rayleigh number This allows us to characterize the conditions of appearance of gravitational convection in the membrane systems
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